Forming thin liquid phase epitaxial layers

ABSTRACT

A thin layer of liquid phase epitaxial melt material (26) is formed on a wafer (15,16). The thin melt layer (26) is held in contact with the wafer (15,16) while the temperature of the thin melt layer (26) and the wafer (15,16) are reduced to crystallize a portion of the melt material thereby producing thin and accurately controlled epitaxial layers on the wafer (15,16).

BACKGROUND OF THE INVENTION

The present invention relates, in general, to semiconductor processingmethods, and more particularly, to a novel way of forming thin epitaxiallayers from a liquid phase solution.

The semiconductor industry has long used crystallization ofsemiconductor material from a heated liquid phase solution to formepitaxial layers on semiconductor wafers. One particular use was to growP-type and N-type epitaxial layers on gallium arsenide (GaAs) wafers.The epitaxial layers typically were grown by using a graphite boat whichexposed a number of wafers to a melt that contained a solvent of meltedsemiconductor material such as gallium (Ga), into which was dissolvedsolutes, such as melted gallium arsenide, and dopants, such as meltedsilicon. The wafers were pushed into the melt thereby covering them witha thick layer, generally greater than 1000 microns, of the melt. Whilethe wafers were covered, the temperature of the boat and melt wasdecreased which caused crystallization of material from the melt ontothe wafers thereby growing epitaxial layers of the melt material ontothe wafers. Cooling of the boat continued until it was cool enough forunloading the wafers. These techniques produced thick epitaxial layers,generally 200 microns or greater, which was thicker than the desiredlayers. Consequently, the wafers were polished by mechanical and/orchemical means to obtain the desired thin epitaxial layers. Suchpolishing was time consuming and increased the costs of producing liquidphase epitaxial layers. Polishing operations often broke the brittlegallium arsenide wafers which resulted in reduced yield of themanufacturing process and further increased manufacturing costs.Polishing of the wafers also roughened the epitaxial layer's surfacethereby reducing the yield of subsequent processing operations whichfurther increased manufacturing costs. Additionally, wafer polishing canproduce stress on the semiconductor material's surface thereby reducingsemiconductor device performance and reliability.

Accordingly, it is desirable to have a liquid phase epitaxial processthat produces thin epitaxial layers, that reduces wafer breakage, thatproduces thin epitaxial layers having surfaces suitable for subsequentprocessing operations, and that reduces the manufacturing costs ofproducing thin epitaxial layers.

SUMMARY OF THE INVENTION

Briefly stated, the present invention is achieved by creating a thinlayer of a liquid phase epitaxial melt material on a semiconductorwafer. The thin melt layer is held in contact with the wafer while thetemperature of the thin melt layer and wafer are reduced to crystallizea portion of the thin melt layer material thereby producing thinepitaxial layers on the wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a portion of a graphite boat used toproduce thin epitaxial layers on semiconductor wafers and is illustratedin an initial phase of the manufacturing process in accordance with thepresent invention;

FIG. 2 is a plan view of a portion of a movable graphite slider that isan element of the boat of FIG. 1 in accordance with the presentinvention;

FIG. 3 is a plan view of a portion of a stationary graphite plate thatis an element of the boat of FIG. 1 in accordance with the presentinvention;

FIG. 4 is the graphite boat of FIG. 1 at a subsequent stage of themanufacturing process in accordance with the present invention;

FIG. 5 is the graphite boat of FIG. 3 at a further stage of themanufacturing process in accordance with the present invention;

FIG. 6 is the graphite boat of FIG. 4 at an even further stage of themanufacturing process in accordance with the present invention; and

FIG. 7 is a cross section of a graphite boat illustrated in a finalconfiguration during the process of growing thin epitaxial layers onsemiconductor wafers in accordance with the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The present invention provides a process that produces thin, accuratelycontrolled epitaxial layers on a wafer from a melt or liquid phasesource. While the invention is described with specific preferredembodiments, it is evident that many alternatives and variations will beapparent to those skilled in the semiconductor art. More specificallythe invention has been described for a particular gallium arsenide waferthat is contained in a particular graphite boat, although the method isdirectly applicable to other III-V semiconductor materials, as well asto other boat structures.

Referring to FIG. 1, a cross section of a portion of a graphite boat 10that is used to grow thin epitaxial layers on semiconductor wafersincludes stationary graphite plates 11 and movable graphite sliders 12that form reservoirs 13.

Referring to both FIG. 2 and FIG. 3, a plan view of a portion of slider12 shown in FIG. 2 and a portion of plate 11 shown in FIG. 3 will assistin understanding the operation of boat 10 shown in FIG. 1. Referringprimarily to FIG. 2, slider 12 is a single piece of graphite that has ashape which is similar to a paddle. Recesses 21 are used to holdsemiconductor wafers, and have a depth into slider 12 that is sufficientto accept a wafer plus a thin layer of melt material that will be usedto grow a thin epitaxial layer on the wafer as will be shownhereinafter. Each opening 22 through slider 12 forms a portion of eachreservoir 13 that is shown in FIG. 1. As will be shown hereinafter,inner perimeter surface 24 of each opening 22 will be used to controlmovement of material in each reservoir 13 (shown in FIG. ensuring thatonly a thin layer of material is used to form thin epitaxial layers.Slider 12 also has an opening 19 through which a control rod is insertedin order to control the movement of slider 12. Referring primarily toFIG. 3, stationary plate 11 also is a single piece of graphite havingopenings 17 through plate 11 which are also used to form a portion ofeach reservoir 3 (of FIG. 1). Each recess 25 is used to hold asemiconductor wafer and has a depth into plate 11 that is sufficient toaccept a wafer plus a thin layer of melt material that will be used togrow a thin epitaxial layer on the wafer. Inner perimeter surface 27 ofopening 17 is used to ensure that a thin layer of melt material is usedto form epitaxial layers as will be explained hereinafter.

Referring once again to FIG. 1, plates 11 and sliders 12 are alternatelystacked to form boat 10. Each slider 12 holds a wafer 16 in each recess21 (shown in FIG. 2) and each plate 11 holds a wafer 15 in each recess25 (shown in FIG. 3). During the process of forming epitaxial layers onwafers 15 and 16, boat 10 is contained in a furnace that has anaccurately controlled temperature which is sufficient to melt III-Vsemiconductor material that is contained in reservoir 13. This III-Vsemiconductor material generally includes a material, such as gallium,that acts as a solvent to dissolve predetermined amounts of a solute,such as gallium arsenide, and dopants, such as silicon, thereby forminga liquid phase solution or melt 18. For example, a melt that containsboth gallium and gallium arsenide that are doped with silicon istypically maintained at a temperature between 850 and 920 degreescentigrade. It should be noted that wafers 15 and 16 could be heated toa temperature that is different than the temperatures of melt 18although this is not the preferred method. Since each slider 12 will bepushed into melt 18, simultaneous movement of all sliders 12 isfacilitated by a control rod 14 which interconnects each slider 12. Inthe preferred embodiment, each wafer 15 and 16 is a gallium arsenidewafer, and melt 18 contains gallium arsenide and silicon that aredissolved into a liquid gallium solvent.

Referring to FIG. 4, as each slider 12 is pushed into melt 18 each wafer16 and each wafer 15 is exposed to melt 18.

Referring to FIG. 5, at the point that each slider 12 is completelypushed into melt 18, each wafer 15 and 16 is covered with a thick layerof melt 18. Previous methods for producing epitaxial layers from melt 18utilized the configuration of sliders 12 and plates 11 shown in FIG. 5to produce thick, approximately 200 microns thick, epitaxial layers onwafers 15 and 16.

It has been found that providing a thin layer of melt material on eachwafer 15 and 16 facilitates forming thin epitaxial layers of accuratelycontrolled thickness. The thickness of epitaxial layers grown fromliquid phase solutions or melts is determined by the thickness of themelt from which the layer is crystallized, the rate at which the melt iscooled, and the diffusivity of the solute in the solvent used for themelt. The relationship of these parameters depends on how far the solutediffuses through the melt solution in a given time period, or thesolute's diffusion length as it is generally referred to in the art.When the solute's diffusion length is comparable to the thickness of themelt used to grow the layer, the parameters are related by the followingequation:

    d=((R.sub.c W.sup.3)/(C.sub.s mD))((Dt)/(W.sup.2)-1/3)

where:

d=epitaxial layer thickness,

R_(c) =rate at which boat 10 is cooled,

W=thickness of the melt used to grow the epitaxial layer,

C_(s) =dopant concentration in the solid form of the melt material e.g.As is 2.21×10²² atoms/cm³ in Ga,

t=time period used for cooling boat 10,

m=slope of the liquidus curve from the phase diagram of the solvent usedin melt 18, and

D=diffusivity of the solute in the solvent.

Consequently, providing a thin layer of melt material (W) on each wafer15 and 16 can be used to accurately control the thickness (d) of aliquid phase epitaxial layer that is grown on each wafer 15 and 16.

It has been found that such a thin layer of melt 18 can be formed bycontrolling the depth of each wafer 15 and 16 in respective recesses 21and 25 (shown in FIG. 2), and by using the movement of slider 12 toremove portions of melt 18 from wafers 15 and 16 thereby capturing apredetermined amount of melt 18 on wafers 15 and 16.

Referring to FIG. 6, in order to form a thin layer of melt 18 on eachwafer 15 and 16, slider 12 is now pulled back toward its' originalposition by control rod 14. During the movement of slider 12, surface 24pulls a portion of melt 18 that is covering each wafer 15 back intoreservoir 13 leaving a thin layer of melt 18 on each wafer 15.Similarly, surface 27 of plate 11 levels melt 18 that is covering eachwafer 16 and pushes a portion of melt 18 back into reservoir 13 leavinga thin layer of melt 18 on each wafer 16.

Referring to FIG. 7, once slider 12 has returned to the originalstarting position, a thin melt layer 26 remains covering each wafer 15and 16. The height of layer 26 over wafers 16 is determined by the depthof recess 21 (shown in FIG. 2) minus the thickness of wafer 16.Similarly, the height of layer 26 over wafers 15 is determined by thedepth of recess 25 (shown in FIG. 3) minus the thickness of wafer 15.The height of thin melt layer 26 can be adjusted by placing spacersunder each wafer 15 and 16, or by other similar means.

Once the desired thin melt layer 26 is formed, a thin epitaxial layer isgrown on each wafer 15 and 16 by uniformly cooling boat 10 at apredetermined rate and for a predetermined time, as determined by usingthe height of layer 26 as "W" in the equation of FIG. 5. In most cases,boat 10 is cooled at a rate between one-half a degree centigrade perminute and five degrees centigrade per minute until it reaches apredetermined unloading temperature. Typically the unloading temperaturemaintains the material in reservoir 13 and the uncrystallized materialin layer 26 in a liquid state thereby facilitating easy removal. Onceboat 10 has cooled to the predetermined unloading temperature, boat 10is removed from the furnace, each wafer 15 and 16 is unloaded from boat10, and uncrystallized portions of layer 26 are rinsed, typically usingHCl, from each wafer 15 and 16. In the preferred embodiment, theunloading temperature is greater than fifty degrees centigrade. The thinaccurately controlled epitaxial layers produced by this process do notrequire mechanical or chemical polishing thereby improving yields ofsubsequent processing operations by providing smooth epitaxial layersurfaces. Both the increased yield and elimination of polishing aid inreducing manufacturing costs of wafers 15 and 16.

By now it should be appreciated that there has been provided a novel wayto form thin epitaxial layers from a liquid phase source or melt onto asemiconductor wafer. Use of a thin layer of melt material permitsgrowing thin accurately controlled epitaxial layers that do not requiremechanical or chemical polishing, thereby reducing the number of brokenwafers and decreasing the wafer's manufacturing costs. Elimination ofwafer polishing also provides smooth surfaces on the epitaxial layerswhich result in higher yield of final semiconductor devices, reducedmanufacturing costs, and improved semiconductor device reliability.

We claim:
 1. A method of growing thin epitaxial layers of galliumarsenide (GaAs) material from a gallium arsenide melt whichcomprises:heating to a first temperature a graphite boat having aplurality of reservoirs that contain a melt including at least galliumarsenide and silicon that are dissolved in melted gallium, having aplurality of movable graphite sliders, having a plurality of fixedgraphite plates, and having a plurality of recesses that have a depthwherein a first portion of the recesses are in each slider and a secondportion of the recesses are in each plate and each recess contains agallium arsenide wafer that has a thickness; covering each wafer withthe melt by pushing each slider into the melt wherein the melt that iscovering each wafer has a thin first section that is within each recessand a second section outside each recess; removing the second section ofthe melt that is covering each wafer by returning the second section ofthe melt back to the plurality of reservoirs while leaving the thinfirst section covering each wafer; and growing an epitaxial layer oneach wafer by cooling the graphite boat at a rate of approximately 0.5to 5.0 degrees Celsius per minute until reaching an unloadingtemperature thereby crystallizing a portion of the thin first sectiononto each wafer.
 2. The method of claim 1 wherein removing the secondsection of the melt includes removing the second section of the meltthat is covering each wafer by pulling each slider out of the melt andreturning the slider back to an original position.
 3. The method ofclaim 1 further including the thin first section that is in each recesshaving a height that is determined by the depth of each recess minus thethickness of the wafer in each recess.